


At Your Core

by ShyneAnon



Category: Undertale (Video Game)
Genre: Alternate Universe, F/M, Like seriously this is meta even for something about Sans, Meta, Metafiction, No Smut, Reader Is Not Chara (Undertale), Reader Is Not Frisk (Undertale), Reader-Insert, There's fluff tho, UF Sans is actually a good guy (sort of I mean he's from Underfell but), and she has DIGNITY and SELF-RESPECT, he's also v insecure :(, reader is female, sort of it's hard to explain
Language: English
Status: In-Progress
Published: 2020-11-26
Updated: 2020-12-02
Packaged: 2021-03-10 05:19:40
Rating: Teen And Up Audiences
Warnings: Creator Chose Not To Use Archive Warnings
Chapters: 2
Words: 4,640
Publisher: archiveofourown.org
Story URL: https://archiveofourown.org/works/27718061
Author URL: https://archiveofourown.org/users/ShyneAnon/pseuds/ShyneAnon
Summary: You're a dimension-hopper and an engineer, and you work at the labs of "Headquarters," a dimension that connects all of the others together. After making some terrible choices in boyfriends while in high school, you quit dating while you got your degree. Nowadays, you know better than you did when you were a teen. Bad boys are bad. Stay away. Miles away. Unfortunately, they're still what some people might call "your type," but hey, you work as a scientist. Scientists are nerds. So there's absolutely no way some asshat skeleton who dresses like a teen going through their goth phase, and has a rather unsavory background, is going to get hired to work alongside you, right?Hold on, that description was oddly specific.
Relationships: Sans (Undertale)/Reader
Comments: 6
Kudos: 34





	1. He's From an AU

**Author's Note:**

> Hi! It's been a long while since I posted fanfic. I've been using this multiverse concept to write fanfiction for years, but I've never actually posted any of those fanfics anywhere. So, I'm hoping you all will enjoy it.

As you finished getting ready for work in the morning, your first instinct was to open the window’s shades. It always was your first instinct-- to illuminate the indoors with sunlight-- even though you’d been living here for a year and you knew there was no sunlight outside. Just a void. You were lucky enough to see stars and galaxies off in the distance, but none were close enough to illuminate the room.

But there was no need to ponder that now. Otherwise you’d be late for work.

You stepped outside your apartment and the door promptly slid shut behind you. You blinked-- again, not at the sun, but at all the artificial lights illuminating the massive space around you. The wall opposite you-- which was very, very far away-- was made of glass, and also had multiple floors, each of which contained a number of quite massive artificial gateways. Portals to other dimensions. You were thankful you didn’t have to bother using them yourself.

You knew the route to work well, so as you made your way to the public transportation on your floor, you recalled how you’d gotten here. You’d discovered the multiverse that lay underneath everyone’s noses as a young teen, and it had inspired your career path. After a degree in engineering you were able to work here, at Headquarters’ labs. There was a reason there was really nothing (or everything?) surrounding HQ’s walls. Headquarters was an inbetween spot, a place that connected every known universe to each other. Most sapients had to pass through HQ to travel places. It was strange, how accustomed you’d become to everything over time.

Once you arrived at work, you used your trainee ID to get inside. The thing was, your degree could only offer so much. Most people from the Motherworld-- your dimension, _our_ dimension-- had no access to the rest of the multiverse, so they couldn’t teach you what you’d need to know here. Nothing about artificial gateways, star fuel, et cetera. So despite your education, you needed training.

“H… hey there, (y/n).”

Alphys. Classic Alphys, specifically. She’d been here longer than you, but she was still really nervous all the time. It seemed it was more of a character trait. You didn’t mind; she was really sweet. You felt a little intimidated by her intelligence sometimes, but hey, even she’d had to go through training.

“Hi, Alphys! How’s work been going with the core?” You specified, “HQ’s core, I mean.” The engineers here had been really fascinated by the Core in Alphys’ universe. You weren’t sure how it worked, but it apparently generated magical electricity, and they were curious about whether that sort of thing could increase the efficiency of Headquarters’ current power source. Engineers were always big on efficiency.

“Um, O… OK, I guess.” She didn’t seem so sure. “I… It’s just weird-- star fuel is such a good source of power, but it’s so different from what the Core back home produces. A-- and since we’re not on a planet of any kind, the whole ‘geothermal energy’ thing is a little bit… useless?”

Then she smiled, one of her mildly nervous smiles.

“B-- but, I heard we’re… getting a new engineer today, and… and he’s from _Undertale_ too! So, h-- he might be able to help.”

You hadn’t heard about this. “Wait, really?” _Wait._ “Is it Sans?” You’d never actually played the game, or met Sans, but there was no way you wouldn’t know who he was. Everyone who had played Undertale-- the universe Alphys came from-- talked about him. And he had been invited to work at the labs before. He’d just turned the offer down. You weren’t sure why they’d invited him to be an engineer, though, considering all you knew about him was that he sat around avoiding work all day, and enjoyed bad puns. He sounded fun, but you figured he probably got bothered by fans all the time, so you hadn’t tried to meet him.

Her smile faltered. “W… well, yes and no. H-- he’s not really from… from my _Undertale_.”

“How do you mean?” You knew there were multiple versions of _Undertale_ , each related to the different endings of the game. As you’d discovered when you were younger, the multiverse wasn’t actually created by timelines splitting every time an opportunity for multiple outcomes to an event rose. Everything branched out from your universe-- the Motherworld, or sometimes referred to as Real Life-- because dimensions were generated from the imaginations of the humans who lived there. The only reason Alphys existed was because of the people (person? you’d never looked it up) who’d come up with her. There were many timelines for _Undertale_ , yes, but it was because those timelines had been written and coded into the game.

“H-- he’s from an… an AU.”

An AU? You tried not to look hesitant. AUs were made up by fans, and sometimes they could get pretty weird. That said, he wouldn’t have gotten the job offer if the employer hadn’t thought he could perform his job properly. “Huh. Which one?”

“Th… this one called Underfell. I-- I didn’t look it up.” She smiled a little. Her tiny smile was cute. “I… I kinda want it to be a surprise.”

You were too curious to resist, so you started to pull out your PDA, a metal cylinder that conjured up a holographic pad when activated. It would have information about Underfell on it-- but you were interrupted by the door to the room sliding open. You and Alphys both turned to greet whatever coworker had walked through the door, but you paused when you realized it wasn’t anyone you knew.

It was Sans.

Well, Sans, but also not Sans.

The guy looked like Sans-- you’d seen artwork of him, because it was freaking everywhere-- but Sans if he were going through his edgy phase far too late in life. As one might expect from that summary, he was dressed entirely in black and red (with some yellow mixed in). He wore a large black jacket with very poofy fur trim; it actually looked really warm and fluffy, and you already wanted to try it on. He had a red sweater underneath, which was honestly a good choice. The labs could get cold. Like Classic Sans, he wore basketball shorts, but he wore sneakers on his feet instead of slippers. Despite his actual body being a lot like Sans’, his face was different: The lights in his eye sockets were red, and he had a series of very sharp teeth. Also, was one of those teeth gold?

He didn’t notice the two of you immediately, instead glancing around-- it was his first day, so he wouldn’t be familiar with the labs-- but when he finished and turned to look ahead of himself, he blinked his eye… sockets. You glanced over at Alphys for a moment. She looked even more jarred than you; that made sense, seeing as she knew Classic Sans personally.

After a second, the new Sans’ expression changed; he smirked and his eyes lidded. “Hey there, sweetheart.”

Later you would wonder if Classic Sans also had a Brooklyn accent, but at the moment, you looked at Alphys again. She sputtered for a few moments before saying “I-- I’m gay,” and then hastily adding, “and I have a girlfriend!”

“Wasn’t talkin’ to you,” he said, and winked. Wait, was he flirting with you? You were human, you hadn’t assumed... Was your face getting hot? Ugh, it was. And he seemed to like that.

He walked over, and Alphys immediately backed off, looking confused and kind of intimidated despite the fact that he wasn’t really tall. “Nice tah meet you. Name’s Sans.” He shrugged. “But you can call me Red. Most people call me that.”

Probably to distinguish him from Classic Sans. And maybe even more Sans-es(?). How many Undertale AUs were there? At least your face wasn’t hot anymore.

He held out his hand. “Got a name, beautiful?”

And it was hot again. You tried to ignore it and shook hands with him. You’d never shaken hands with a skeleton before, and you tried to distract yourself from your embarrassment by noting the differences of a hand that had no skin or muscle. Thankfully, you didn’t forget to tell him your name.

“Pretty name,” he said in a low voice, and you realized your hands were still clasping each other. You pulled your hand away, though he didn’t seem to mind.

“Thanks,” you managed.

“Um,” said Alphys-- it was obvious she felt uncomfortable. “I… I gotta go to--”

One of your bosses walked in-- this one was human, as opposed to the many extraterrestrials you had in the labs. You thanked your stars for his appearance; you weren’t sure how to respond to Red’s advances. Surely this guy wasn’t the new engineer. He didn’t look or sound like a scientist in the slightest. Scientists were nerds, it was why you’d been thankful to enter this line of work.

“Hey there,” he said-- to Red. “You’re Sans, correct?”

“Yeah," was his response. "I go by Red t’most people outside my dimension, though.”

Your boss greeted him and then shook his hand. You couldn’t help but speak up:

“So Red is the new engineer?”

“Yep,” your boss responded. “Pretty new to HQ in general, I think.”

Red nodded to confirm. “Wasn’t really interested in visiting till I got the job offer.” His grin got very cheeky. “Well, I wasn’ interested then either. This is jus’ an excuse to get my ass away from Boss.”

You were both surprised and not surprised by the swear. It wasn’t a surprise that he cursed, just that he’d cursed in a work environment. Strangely, your boss didn’t say anything about it. Instead, he turned back to you. “He’s a trainee, like you.”

Red’s eyelights flitted over to you and he smiled. “Oh hey, we’re gonna be workin’ together?”

“That you are. In fact, she could probably show you around to start the day off. How’s that sound, (y/n)?”

You nodded. “Sure, that’d be great.” You kept your _“You’ve gotta be kidding me”_ to yourself. It wasn’t that you… disliked Red. In fact, that was the whole problem.

You had a sort of… personal history with, as most would call them, “bad boys.” You’d dated a lot of them while in high school, and you’d come to regret it. And these weren’t just moody guys in leather jackets who drove past the speed limit and got suspended from school every now and then. These were aspiring criminals you’d been dating, which had shockingly turned out to be a horrible idea. Who would have thought? But at least you’d learned your lesson, albeit after putting up with far too much toxicity.

That being said, your attraction to their type hadn’t really ever gone away, even when objectively you knew they were bad news. But you’d figured that the job you’d aspired to-- one in a scientific field-- would basically guarantee you wouldn’t have to really interact with those kinds of men. At least, not regularly.

And then there was this asshole, who gave you another wink when your boss wasn’t looking. If he tried anything you were going to report him, but he probably knew that. You were about 99% sure this version of Sans had been primarily created by thirsty fangirls who had a thing for the bad boy character archetype.

Not that you were any less guilty.

You turned back to Alphys, who looked less uncomfortable but had turned to her phone to occupy herself. “Hey Alphys,” you said, and she looked up. “Good luck working on the core.”

She smiled. “S… sure! Thanks! Good luck, with… u-- um….”

“Red,” said Red, giving her a surprisingly friendly grin, although it still had some of that edge to it. Was he just faking it in front of your boss, or was he usually pretty friendly when he wasn’t flirting with someone?

“Thanks,” said your boss. “You guys have a good day, alright?”

Everyone else wished him the same, and he hurried off. He probably had lots of stuff to do. Once he was gone, Red gave you another grin. This one was thankfully on the cheekier side.

“I didn’ know you were a trainee.”

“Yep,” you said. “I got hired less than a year ago, and there’s a lot to learn.”

“A lot, huh?” He gestured around. “Well, then, gorgeous. Enlighten me.”

You hated this guy.

Well, not really.

Again, that was the issue.


	2. Misconduct

**Summary for the Chapter:**

> Red isn't used to a professional environment.

You waved Red towards your right and the two of you headed into the labs. There was legitimately a lot to see here. “So,” you said, “you’re seriously an engineer?” It was a dumb question, but he really didn’t look the part.

“Quantum engineer, yeah. Don’t  _ technically _ have a degree in physics, but I may s’well. You?”

“Where I come from, quantum engineering isn’t really much of a thing. So I got an ‘engineering physics’ degree.” You used air quotes.

“Not a thing, huh? Ya must have a lotta trainin’ to do.”

“Yeah. It’s kind of like extra school, but hands-on.”

The two of you nearly bumped into a robot as it came hurtling around a corner. It apologized profusely before leaving at top speed.

Red gestured after it. “Ya got sapient robots here too?”

“We’ve got basically everything you can think of,” you told him. “Good thing, too. If it weren’t for the aliens we wouldn’t have this place running on star fuel.”

“Yeah?”

“Yeah, their civilizations are higher on the Kardashev scale, and they’ve been nice enough to share their tech. Basically this whole place is one giant Dyson Sphere.”

“No idea whatcha just said, sweetheart.”

His tone of voice when he said it was funny, like he was happy to be confused. Or, happy for you to confuse him…? No, that didn’t make sense either. It was also weird that him calling you  _ “sweetheart” _ didn’t make you uncomfortable. You smiled. “Ask the aliens, I’m not going to explain everything to you.”

He grinned, but it seemed flirtatious again. “Yeah? You’re a busy lady, huh?”

“Very busy.”

“Too busy for me?” A wink. “Cuz I’ll never be too busy for you.”

Your face was hot again! You hated that it did that! It was clearly visible to him and he clearly enjoyed the fact that you were flustered.

“I like that color,” he said, his tone lower and a bit more suggestive.

You managed to fight back your body’s impulse to shudder at his voice, and instead folded your arms, doing your best to look annoyed. “No thanks. I’m into nerds.”

“I can be a nerd if ya want.”

He stepped towards you, and while the motion wasn’t inherently threatening, your instinct was to step back. Still, he continued to close the distance, with you trying to maintain it until you lightly bumped against the wall.  _ Come on. Seriously? _

The look in his eyes was that of a predator looking down at its prey. The lights in his sockets looked you up and down, and the hungry smirk that was already on his face grew, giving you a better look at his sharp teeth. He placed one hand against the wall next to you. Had he forgotten where you were? You were at  _ work! _ There were probably cameras! You glanced around, hoping nobody was about to walk in and think the two of you were planning on having a makeout session. Now that he was close, you actually noticed that his… skin? Bone?... was littered with small nicks and scars.

You had two different inner reactions to this. The sensible part of your brain immediately started sounding alarm bells and sent a flurry of concerns running through your mind so fast you could hardly process them.  _ Not good. This guy is bad news. He probably gets into fights. He’s probably violent. He probably has anger issues. Abort. Abort. Push him off. Tell him not to ever do this again. _

Your hand lifted slightly, making to push him off, but your breath hitched as you felt his free hand gently run down your side and settle on your waist. The tension in the air was so thick that you felt glued in place. You couldn’t move. Crap, you probably looked so small and timid, frozen in place and your face undoubtedly red.

~~ You wondered for a moment if he liked that. ~~

The other part of your brain— the less sensible part— looked at the scars on him and felt a wave of sympathy.  _ Who did this to him? Why? Is he OK? Does he have friends? Anyone who cares about him? I hope so. _

It also looked into that confident, almost possessive gaze and that fanged grin, and… Agh, you were an adult, you should’ve known better than to find this attractive. You were. At  _ work. _ This was severe misconduct.

Who were you kidding? Breaking the rules only made it hotter.

Man, you had some serious problems.

His crimson eyelights roamed your body again, slowly, as if he were making sure to take everything in. He made a growl of approval before leaning forward; for a moment you thought he was trying to kiss you, and, well, even deer-in-the-headlights you had limits. You quickly put a hand on his shoulder, ready to shove him off, but paused when you realized his mouth was next to your ear. Did he have something to tell you?

_ It doesn’t matter. Push him away. _

_ But those eyes… That smile…. _

As if he could hear your thoughts, he let out a low chuckle. You swallowed thickly.

Finally, he purred in your ear, low and quiet:

“ _ Mass is the resistance a body of matter offers to a change in speed or position upon the application of a force. _ ”

You tried to hold back any reaction.

You snorted.

Before you could do anything about it, you were laughing-- hard. Red immediately backed away, beaming.

“Hey,” he said. “Why does a burger have less energy than a steak?”

You were still snorting, and you refused to encourage him by asking why. He continued anyway:

“Because it’s in its  _ ground state.” _

More snickering. You had to stop, your laughter was only egging him on.

“Why can’t ya trust an atom?”

“Please stop,” you said, but you were smiling. You were glad you weren’t actually supposed to be working on something right now.

His grin brightened. “They make up everything.”

You whacked him in the arm, and he chuckled a little. “Stop,” you managed, “I’m supposed to be showing you around!”

He leaned close to you again. “Not my fault you’ve got a sexy laugh.”

Why did he have to make his voice such a low growl when he flirted with you? You were pretty sure your face was on  _ fire. _ “Soooo this is the biomedical research wing,” you said, scanning your hand to get access. You noticed your body was a little shaky and you could only hope it wasn’t noticeable to him.

“Aww. No more flirting?”

“We weren’t flirting.”

“You sure, doll?”

“We’re coworkers.” You didn’t mention that here at Headquarters, people didn’t have the same aversion to the idea of dating a coworker. Only if one of you was the other’s boss. Power dynamic, and all that.

“Can I get in trouble for it?”

Well, you weren’t going to lie. “Not unless I report you for workplace harassment.”

His smile faltered. “Am… am I harassing you?”

You backtracked. He wasn’t making you uncomfortable. “No, no, it’s not like that.” Turning to him completely, you decided to clarify, “I mean, the whole pinning-me-to-the-wall thing was super inappropriate behavior, even as a joke, so don’t… don’t do it, but I… You’re not making me feel gross.”

What you meant was  _ “Don’t do it, but I really really like it,”  _ but you weren’t going to tell him that.

You decided to add a preemptive warning: “Also, don’t… make any suggestive jokes.” You resisted the urge to add that you only meant not to do it  _ at work. _

His grin returned. “What?  _ Me? _ I’d  _ never. _ ”

_ Why? Why why why did they hire him? _

“You gotta stop blushin’ like that.” The grin widened, the fluorescent lights in the ceiling glinting off his gold tooth. ~~Why were the sharp teeth hot?~~ “It’s almost like yer enjoyin’ this.” His eyes lidded. “But no, not you.”

He leaned close again.

“You’re a  _ good girl. _ ”

He lowered his already baritone voice for that last sentence and... _ I should be offended right now, I should be  _ offended _ , why can’t I just be offended like a normal person? _

Maybe because it was a bit funny. He probably thought you’d always been as level-headed as you ~~tried to be~~ were now. He had no idea what you’d been like in high school.

You kept walking forward, trying to ignore your rising temperature again. “What did I  _ juuust _ saaaaay?”

“But ya like it.”

Your eyes narrowed.

“I ain’t hearin’ a protest.”

You did your best to give him a serious look. It wasn’t really working, you could tell, but he surrendered, his grin actually looking a bit more awkward.

“OK, OK, I got it. We could get in trouble. No more.”

A moment of awkward silence.

“... Sorry. This is kinda new fer me. We weren’t exactly worried about ‘workplace harassment’ where I come from.” He made air quotes, as if the term was vaguely foreign to him. Well, he was apologizing. You weren’t accustomed to that from people like him.

“Where do you come from?” you asked. “I mean, I know an  _ Undertale _ AU, but what’s the twist with yours?”

“The... the Underground was ugly in my dimension. Lotsa… killin’ and… worse than killin’.”

You knew what the Underground was thanks to Alphys, so you understood. He didn’t seem like he wanted to talk about it, so you just said, “I’m sorry.” This explained his behavior, though.

“Hey, I’m outta there, an’ the other Sans-es are tryna help me… y’know, not be an asshole.”

So there were a lot of AUs, then. But before asking about that, you wanted to address the self-deprecation.

“You’re not an asshole,” you said, smiling. “You’re just… very,  _ very _ forward.” You could feel your smile grow into a grin as you said it. It was actually amusing, now that you weren’t worried about getting in trouble.

“Ya like forward?”

“ _ Red. _ ”

“OK, OK, sorry.” He raised a brow. “Where’re you from?”

You smiled. “I’m from the Motherworld.”

He froze, and you realized you were standing in the middle of the wing. You hadn’t shown him anything. It was his fault! For being distracting!

“You serious?” he asked.

“Yeah.”

“So you can… Can you open gateways all by yerself? No machine.”

“Yeah.”

“Shit, for real? Couldja show me?”

The look in his eyes… This was a moment where you could see the scientist in him and it felt… relatable. You smiled and headed over to some tanks housing strange fauna. Sitting down in one of the chairs next to them, you held your hand out. Red eagerly followed so he could get a better look.

A small, glowing orb of light formed in your palm-- the beginnings of a wormhole. Red was awestruck, and tentatively reached out to touch it. It was safe, so you didn’t stop him. Since it wasn’t an opened gateway, his hand slipped right through the light the way it would through a liquid, the light swirling around his hand. He let out a small laugh that sounded like it was less of amusement and more of wonder. He then gently began to run his fingers-- or, phalanges-- across your palm, as if he were looking for some sort of mechanism that allowed you to do what you were doing. They traced across your own fingers, and he gently took your own hand. It felt funny, but soft, and… nice, so you didn’t stop him.

Eventually he looked at you, and as you made eye contact, you realized the light in his eye sockets had… gotten bigger? Dilated.

“You’re amazing,” he said.

Before you could react in any way, there was a loud whump as one of the strange animals threw itself at the glass, probably perceiving you two as having invaded its territory or something. You both jumped.

“We should probably go before it tries to break the glass,” you said. Not that the chances of it breaking the glass were high.

“Y-- yeah,” said Red. Had he just stuttered? You looked over at him to find that his face had turned a red color. Skeletons from his universe could blush? You wanted to ask, but you didn’t want to call him out, so instead you asked a broader question.

“Do you guys have blood?” Didn’t bones have blood in them? Sort of? In the marrow or something?

“We’re made of magic. Monsters are.” His blush was starting to fade.

“Monsters from your dimension?”

“Yeah, tha’s what I meant.”

Despite the blush being gone, he still looked uncharacteristically shy. So you said, “Hey. Wanna see the core? It’s a star.”

He looked at you. “Whaddaya mean?”

“I mean we have this whole place built around a star and it’s drawing its energy from the star. And part of it is inside the lab. You wanna see it?”

“See a fuckin’ star up close? Hell yes.”

You smiled. “Then follow me.”

As you started to lead him through the labs, towards the center, he thought of something else. “I heard ya don’t have a soul? Souls are an  _ Undertale _ thing.”

That didn’t make any sense. “Who told you that?”

“Some’a the other Sans-es.”

“I mean, what do you mean by soul?”

“Th’ thing that represents ya that ya bring out during fights. And, um…” He trailed off, turning red again.

You were hesitant, but you quietly guessed, “Sex?”

“N-- no, not like that! It’s hard tah explain.”

OK, you wouldn’t push him. “I guess our souls are more metaphysical. Like an idea. They can’t really manifest physically.”  _ Unless… _ “I mean, if they’re a physical manifestation of who you are, maybe they could manifest physically in your world. Just not here.”

He didn’t respond besides a shrug. Maybe he wasn’t sure how.

Curious, you asked, “What else have you heard?”

He studied your… arm? Back? For a moment. “D’you guys really have wings?”

You smiled and spread them; they always vanished when you weren’t using them. “Yep. It’s a dimension-hopper thing. Everybody’s wings are different.” They could look like anything; yours actually looked as if they were artificial-- advanced machinery that had some scars of its own. Kind of like Red’s scars, actually.

He stared. “Damn, those look badass.”

You gave him a playful grin. “Thank you, I know.”

His smirk from earlier returned, something that really shouldn’t have surprised you.

“Y’know,” he said, “I heard some rumors about yer wings.”

His hand began to reach up to touch one and you made them disappear. Your face had gotten warm again. “That only applies to some of us.”

Another, more smug grin. “Yeah? Like you, maybe?”

“I never said that.” Thankfully, you’d reached the entrance to the Dyson Sphere, which everyone had begun calling “the Core” when Alphys had arrived. She’d kept calling it that, and then everyone else had sort of picked up on it. Red was still pressing about the wings being erogenous, but when you said “Hey, you’re about to see the star,” he quit.

“Do I need… glasses ‘r somethin’?”

“Nah, the material around it is something from… one of the extraterrestrials’ universes.” You weren’t sure which anymore; it was easy to get them all mixed up. You placed your hand on the scanner.

The door slid open to reveal a bright light.

  
  


**Notes for the Chapter:**

> What's sexier than physics, amiright?
> 
> I had way too much fun writing that scene.


End file.
